Field of Endeavor
The present invention relates to magnets and more particularly to bulk exchange spring magnets.
State of Technology
The energy density (or energy product) of a magnet is the amount of useful magnetic work that can be extracted from a magnet and is a function of the remanence and coercivity of the magnet. Exchange spring magnets (ESM) are metamaterials consisting of magnetically soft particles with a large remanence, such as iron or permendur—intimately coupled to hard magnetic particles such as SmCo5 or Nd2Fe14B. The resulting composite benefits from the best properties of its constituent materials to form a magnet with a superior energy density. While the best magnets available today have energy densities ˜400 kJ/m3, the upper limit on a well designed ESM approaches 1 MJ/m3.
The challenge in producing high performing ESMs has been the inability to precisely control the spacing of the particles and the coupling between them. Electrophoretic deposition (EPD) is a processing method which utilizes the induced surface charge particles exhibit when placed in both aqueous and organic liquids. The surface charge is then used to control the motion of the particles in suspension in the presence of electric fields. As such, EPD is the particle level equivalent of electroplating and permits the precise control of particles needed to manufacture superior ESMs with energy products approaching the theoretical maximum.
U.S. Pat. No. 7,344,605 for an exchange spring magnet powder and a method of producing the same provides the state of technology information quoted below:
“As related permanent magnet materials, ferrite magnets which are chemically stable and inexpensive and rare earth metal-based magnets having high ability are practically used. These magnets are constituted of approximately a single compound as a magnet compound, and recently, exchange spring magnets are noticed which are obtained by complexing a permanent magnet material having high coercive force with a soft magnetic material having high magnetic flux density.”“Such exchange spring magnets are expected to have high maximum energy product, and theoretically, extremely high magnetic property of 100 MGOe (.apprxeq. 796 kJ/m3) or more can be realized.”
U.S. Pat. No. 6,736,909 for a bulk exchange-spring magnet, device using the same, and method of producing the same provides the state of technology information quoted below:
“In general, the structure of the exchange-spring magnet is composed of a plurality of laminated thin films of a hard and soft phase or of the soft phase composed of fine grains dispersed in basic structures of the hard phase, and is termed as a nanocomposite structure. The presence of the laminated structure of the thin films or the dispersed structure of the fine grains in a macrostructure results in mere coexistence of the hard phase and the soft phase in the magnet structure with a demagnetization curve, which represents the magnet properties, tracing a snake profile. When, however, the nanoscale domain is composed of the laminated structure or the grain dispersed structure, the magnetization of the hard phase is strongly restricted with the magnetization of the soft phase such that the nanoscale domain entirely behaves as it were a single hard phase. That is, when the exchange-spring magnet, wherein magnetization is aligned in one direction, is applied with the demagnetizing field in a negative direction, a reversal in magnetization occurs from an intermediate portion of the soft phase, with the magnetization, in the vicinity of the magnetic domain wall between the hard phase and the soft phase, remaining in its aligned condition in a positive direction owing to a strong exchange-force. Under such a condition, if the demagnetizing field is released, the magnetization returns along the demagnetization curve. Since this action is resembled to a spring action, the magnet is termed an exchange-spring magnet. Also, the word “exchange” is employed as an initial because its theory is based on an mutual exchange interaction.”“For example, it is considered below about a strong magnetic composite wherein an axis of easy magnetization is oriented in one direction and the hard and soft phases are alternately laminated. When magnetically saturating the composite in a positive direction and subsequently applying the demagnetizing field to the composite in a negative direction, the magnetization is first reversed at the center of the soft phase. At the boundaries between the hard and soft phases, the magnetization of the soft phase is hard to be reversed because the orientation of the magnetization at the soft phase is restricted by the orientation of the magnetization of the hard phase owing to the exchange interaction with magnetic moment at the hard phase. While the magnetic moment at the hard phase may be slightly varied in orientation of the magnetization at the boundaries between the hard phase and the soft phase, the presence of the smaller magnetic field in the magnetization of the hard phase than that of the boundaries wherein the magnetization is irreversibly reversed allow the applied magnetic field to be returned to a zero state such that the system is subjected to a spring back to its original state. If the hard phase is applied with a greater magnetization than the magnetic field that is irreversibly reversed, the magnetization of the entire system is also irreversibly reversed such that the system is saturated in the negative direction.”“In general, what the maximum energy product of the magnet is limited depends on the magnetization of the compound which functions as a main phase. The nanocomposite magnet has shown to theoretically surpass the limit of the performance of the magnet, which has been currently in practical use, such that the nanocomposite magnet surpasses the theoretical value of the maximum energy product of 120 MGOe (about 9.6 MJ/m.sup.3) of anistropic multi layers.”“For all of these various reasons, the spotlight is focused on the exchange-spring magnet as a new magnetic material. The exchange-spring magnet has been usually developed mainly for the compound system composed of a hard phase containing a Nd—Fe—B system or a Sm—Fe—N system and a soft phase containing Fe—B or Fe—Co compounds. Japanese Patent Provisional Publication No. 2000-208313 discloses a technology for obtaining an anistropic exchange-spring magnet powders in finer grains with superior magnetic properties by repeatedly implementing an amorphous processing step and a crystalline processing step.”“As discussed above, the exchange-spring magnet theoretically tends to have the extremely high maximum energy product, though implementation of a full dense treatment of the exchange-spring magnet powders causes the exchange-spring magnet powders to be coarse in grain size at such a high sintering temperature of 1000.degree. C. required in the related art technologies, with resultant remarkably degraded magnetic properties (i.e., the maximum energy product). Therefore, it becomes difficult for the exchange-spring magnet powders to be densified in full dense state while maintaining the finer grain sizes of the magnet powders. Accordingly, in order to avoid the coarse grain growth, an extensive study has been conducted to apply the exchange-spring magnet powders to a so-called bonded magnet (in other word, a so-called plamag, plastic magnet or rubber magnet) wherein the magnet powders are mixed with plastic resin or rubber, followed by solidification of the magnet into a desired profile.”